Method for purifying radioactive waste liquid



United States Patent Gfifice 3,167,504 Patented Jan. 26, 1965 This invention relates to a method for eliminating radioactive isotopic material from radioactive waste liquids and relates especially to the elimination of radiocesium from such liquids by adsorption.

The handling and disposal of radioactive wastes is a constant problem in nuclear energy operations. As the utilization of nuclear energy increases, the safe and economical disposal of radioactive waste becomes more pressing. v

Most radioactive waste material is in liquid form and it is obtained, for example, by dissolving spent fuel in nitric acid. Many of the fission products present in the waste liquid have a short half-life and, therefore, their safe disposal presents no problem. The difiiculty comes about as a presence in the waste liquid of radiocesium (Cs and radiostrontium. I

Radioisotopic elements in radioactive wastes are ultimately safely disposed of by burial in the ground, ocean or other geological formations of the suitable composition. The concentration of radioisotopic elements in liquid waste, especially low level waste, is too low to permit the economic disposal of the entire liquid by direct lburial. Therefore, the liquid waste must undergo preliminary treatment so as to concentrate the radioactive values in the liquid. Various methods have been suggest .ed to efiFect this result, as examples of which may be mentioned: evaporation of the liquid; fixation of radiot isotopic elements by solids; precipitation of radioisoto'pic elements from the waste liquid; and calcination of the .waste liquid. Some of these pretreatments of radioactive waste liquids, especially low level liquid wastes, are

not economically feasible when carried out on large scale.

Thus, for example, concentration of wastes by evaporationis notipractical for handling large volumes of low level wastes.

Oneof the most practical approaches to the disposal of tive elements in the liquid waste on a solid, as by adsorpdioactiveelements. Montrnorillonite, illite, clinoptilolite liquid radioactive waste liquids is the fixation of radioac- 'and erionite have been recommended for fixing radiocesi .um. All of .these' minerals are crystalline silicates. :Montmorillonite and illite are clay minerals; clinoptilo lite and erionite,"wh ich. are most promising, are naturally .o ccurringQZeoIiteSL However, naturally occurring minerals generally have Many -minerals, "such 7 naturally occurring; zeolites, aredefinite limitations'as adsorbents unless they can be eco- 'nomically processed-toassure uniform purity and, there- .fore, consi'stent performance. Thus, mineral composi; -.t ion variesj in many instances because of the presence of iimpuri tiesivhich' frequently cannot be removed easily aur'ingr raeessiiig. :Some minerals cannot be putinto'the desired physical form andjstill maintain sufiicientstability.

of r elativelyl uniform "pure composition.. Therefore,

naturally occurring'nii e'r alsfs'iuch as zeolites have very Y radioactive elements. 1 1

Further objects and advantages will be apparent from 1 a description of this invention which follows.

After extensive experimentation with the adsorptive properties of scores of silicates, including various naturally occurring and synthetic silicates, we have discovered that a particular type of synthetic zeolitic silicate, which may be readily prepared from abundant raw material in the desired purity, possesses exceptional adsorptive selectivity for radiocesium ions. Further, we have found that selectivity for radiocesium ions is not aproperty of zeolites as a class.' In fact, this property apparently cannot be predicted merely on the basis of the chemical analysis of a zeoii-te; Further, wehave also discovered that no direct correlation generally exists between baseexchange capacity or surface area of a zeolite solid and its effectiveness as a selective adsorbent for cesium. Thus, for example, a synthetic crystalline silicate of the approximate empirical formula V Na O.Al O .2SiO .4.6H O

and having an exceptionally high base-exchange capacity of about 660 meq./1OO gm. was found to have a cesium adsorption value (defined hereafter) of only about 21% after 48 hours. This cesium adsorption value did not differ significantly from the cesium adsorption value of another synthetic hydrous sodium 'aluminosilicate of the same Na O/Al O /SiQ mol ratio which had a muchlow- .er base-exchange capacity of 183 meq./ gm. On the other hand, a diiferent sodium aluminosilicate having substantially the same base-exchange capacity as the secondmentioned sodium aluminosilicate was found to be more than four times as effective in adsorbing cesium cations as was either of the aforementioned hydrated sodium aluminosilicates.

Briefly statedjin accordance with this invention, aqueous waste liquids containing radiocesium in solution, such as, for example, liquids ob-tained by leaching spent uranium nuclear fuel with acid solutions, are directly contacted with particles of ahydrous synthetic amorphous T zeolite containing about 4 to 14mols of Si0 per mol of Na O and about lto 2 mols of A1 0 per mol of Na 'O, said zeolite having been obtained by precipitating oxides of sodium, aluminumand silicon in intimate association 'with' finely divided particles of pre-precipitated silica. After being contacted with the zeolite in this mannerfor a time suflicient for thezeolite to adsorb substantially completely all of the radiocesium in the waste liquid, the

cesium-depleted waste liquid maybe discharged to waste or, in some instances, it may be further processedfor removal of other radioactive k elements before disposal. The zeolite withsorbed radiocesiumcan be sealed in a suitable container which is then buried in a suitable geo- "logical formation. A Y

Th" process appears to'be especially applicable to the I purification of'lowieveli waste 1iquids, espe cially substantiallyiifleiltral waste liquid containingan excess of sodiumfions overradioc'esium-;ions,' sincethese liquids do I not lend themselvesto .disposal'procedur'es which are fea- 'sible with waste liquids having a'higher concentration of I prepared by the precipitation process described in US.

Patent No. 2,848,346 to Bertorelii, to which reierence'is made herein. As described in the Bertorelli patent, the production of these unique zeolites involves the following essential steps: (1) forming a suspension of fine free pre ,cipitated silica in a dilute aqueous solution of, a salt of aluminum and a mineral acid; (2) forming a dilute aqueous solution of sodium silicatehaving a mol ratio of SiO to Na O of about 1 to about 3.6; and (3) combining these two liquid compositions with agitation while keeping the reaction medium alkaline so as to precipitate the desired zeolite which is then separated from the reaction medium, dried and sometimes pulverized.

Step'(l), viz.,' the formation of a suspension of precipitated silica in dilute aqueous solution of aluminum salt, is conveniently carried out by decomposing kaolin clay (a mineralof the empirical formula Al O .2SiO .2H O) with sulfuric acid. This results in the formation of aluminum sulfate and a silica residue of the starting clay.

Many methods for decomposing kaolin clay or calcinedkaolin clay with acid are known and methods other than those described in the above-mentioned patent to Bertorelli can be used. Kaolin clay of sufiicien't purity is in extremely abundant supply as compared with naturally occurringzeolites such as clinoptilolite'anderionite and, therefore, the supply of the ultimate zeolite which can be produced from kaolin clay is very great as compared with the Suppl of available naturally occurring zeolites.

The mol ratio of 'SiO to Na O in the composition is varied by changing the quantity of alkali metalsilicate solution ("2) mixed with suspension (1).

Using kaolin clay which has been substantially completely decomposed by acid into analuminum salt of a mineral acid and a silica residue, the rnol ratio of A1 'to Na o inithe zeolite product will be about 1:1. The

mol ratio of A1 0 to Na O in the product can be increased by using concentrated sulfuric acid as the mineral acid and calcining the sulfuric acid reactedclay'intermediate product so asto decompose partially the aluminum sulfate constituentof the reaction product producing alumina and volatile oxides of sulfur-which are withdrawn from the system. The nonvolatile residue obtained by calcination of the acid reacted kaolin clay, consisting of a'rnixture of aluminum sulfate, alumina and silica residue of kaolin clay is'slurried in water in the usual manner. The slurry is mixed as described above with sodium silicate, using the sodium silicate'in amount sufiicient to reactonly withthe aluminum sulfate in the slurry.

Synthetic amorphous zeolites useful in carrying out our adsorptive contact process mayco ntain from 4 to 14 mo'ls of SiO 'per mol of Na Of Zeolites having from 81012) 55 mols-of SiQ per mol of Na O have been found to have? exceptional j'adsorptive. selectivity for radiocesium.

'basis.) r V A slurry of the acid reacted clay was prepared by dispersing 905 gm. of thereacted clay in 1750 gm. of dis- The type of zeolites obtained bythe generalprocedure described above is amorphous to X- rays, in distinct c011 for use as cesium adsorbentswhich. are crystalline The.

surface: area values of the zeolites lie withinarelatively 'trast tonatural silicate minerals previouslyrecommended .60; base-exchange capacities of theamorphous zeolites which are used in carrying out the adsorption process of this I invention vary somewhat with composition, especially with the SiO /Al' O ratio of the z'eolite and are typically within therange of about 180 to 230 med/100 gm; The

'broad.range, also depending on composition, andlrnay be c i Our adsorptive contact process can beicarried out at;

and, generally speaking, contact periods of more than about 12 hours'may not effect a significant improvement The zeolite may be in the in radiocesium adsorption. form of the precipitated powder or the powder may be pelleted by means known to those skilled in the art to form granular particles of the desired'form and shape.

Following are examples-which are illustrative of the practice and utility of this invention. a

I Synthetic amorphous zeolites' withinthe scope of' this invention were tested for cesium adsorption in the presence of interfering ions and compared to several natural materials (clinoptilolite, illite, grundite and erionite) that have shown promise as adsorbents for Cs The composition of the test waste aqueous solution was made up to simulate an actual low level radioactive waste liquid.

PREPARATION 'GF SYNTHETIC ZEOLETE SAMPLES FROM KAOLIN CLAY Georgia kaolin clay was mixed with 190% acid dosage of 93% sulfuric acid and the mixture placed in a vented igas fired oven at a bedtemperature of 600 F. for 6 hours; This product had a V.'M. of 68.4%, an S0 content of 32.9% and a free acid content of 6.2%, weight basis. (The term V.M. refers to volatile matter which is the weight percentage of a material eliminated when it is heated to essentially constant weight at about 1.800 F. Acid dosage is theweight of acid based on the volatile free weight of the clay, expressed on a percentage tilled water F.) containing 48 gm. NaOH to neutrali z'e the free acid. The slurry was cooled, then diluted with distilledwater tornakea slurry containing approxirnately 11% by weight of total aluminumand silicon (expressed as oxides).

' A solution of 19.5% by weight of sodium disilicate (Na OZSiO was prepared by diluting type 0 brand sodium silicate solution in an aqueous solution of NaOH. 1930 gm.of the sodium-disilicate solution was slowly 6*added to ZSOOml. of agitated distilledfwater until apI-I of 10.5-11.0'wasobtainedi 11005 gm. of the slurry of acid reacted clay was'addedto the diluted sodium disilicate .solutionat a rate such that the pHof the resulting slurry was maintained Within these limits. After] all of the slurry was added, the reactionwas allowed to continue for 15 minutes after which'the pH'of the reactionmedium was 9.03102. The slurry was filtered and theprecipitate washed withdistilled water until a sulfate free filtrate was obtained.- The cake was dried in'a forcedair oven at 220 JP; The product w as micropulverizedusinga 0.020 inch screen. it I I i The productfidentified asSample No-2) had the fol lowing analysis (alloxide's are expressed on'a volatile free basis):

I -.r rs m A1203 SiO 76.42 Na O 8.73

ill ree 'molsture, I determined; xby heut ng th essentially eohstantweight at ago v e product: to

(b) A zeolite of the approximate formula was prepared by the general procedure described above with the exception that the acid reacted kaolin clay had been calcined at 1200 F. for 1 /2 hours before being slunried with water, thereby eliminating mineral oil which had been used as a reaction medium in forming the mass of acid reacted kaolin clay.

The resulting product (identified hereafter as sample No. 3) had the following analysis (all oxides reported on a volatile free basis):

. TESTING ADSORBENTS FOR CESIUM ADSORPTION The method used for testing cesium adsorption was patterned after thatused at the Health Physics Division, Oak Ridge National Laboratory, United States Atomic Energy Commission and was as follows: A stock solution was made up to simulate radioactive waste liquid by incorporating 6.3 mg. of purified CsCl in 1 liter of 0.6 N NaNO (reagent grade). To this stock was added approximately 2.5 microcuries of radiocesium. The radiocesium was in the form of a dilute aqueous solution of the chloride salt containing 45 microcuries of radiocesium in 8 ml. of 1 N HCl and was made up by dilution of Cs Cl in the 1 N HCl.

In testing the adsorptive capacity of solid adsorbents for radiocesium adsorption, a 1.6 gram portion of sample of powdered adsorbent was added to 40 ml. of the stock solution in a 50 ml. glass-stoppered volumetric flask. The adsorbent and stock solution were shaken for 30 to 100 minutes and then allowed to stand overnight in the stoppered flask. The next morning a 1 ml. portion of the supernatant solution was withdrawn and was counted three times to determine the Cs content. Each sample was again shaken for approximately one hour and allowed to stand for two days. A 1 ml. portion of supernatant was removed from each sample and another count was made.

The plateau for the scaler and Model 163 Scaling Unit was determined with three different standards (Cs rod, Cs disc and U glass) in place in the detector, a DS55 Scintillation Detector, a well-type instrument with a Tl-activated NaI crystal. The working voltage was found to be 1200 to 1250 volts. The detector was shielded with a lead case especially designed for it. The scaling unit was used at integral counting for the plateau determination, but for the sorption studies differential counting was employed, the discriminator giving a good background of only 8.5 counts per minute.

No detectable adsorption of cesium by the glass-stoppered volumetric flasks was found. The largest error in the results appears to be the standard statistical error due to the magnitude of the counts recorded in the radioactivity determinations.

The results of the cesium adsorption determination and other data on the adsorbent samples are shown in the accompanying table. Surface area measurements were determined by the nitrogen absorption method described by S. Brunauer, P. H. Emmett and E. Teller in their article entitled Adsorption of Gases in Multi-Molecular Layers, on page 309 ofJ.A.C.S., vol. 65, April 1944. Base-exchange capacities of the samples were analytically determined by the ammonium acetate exchange method. The composition of reference minerals was checked by standard X-ray powder diffraction procedure.

The adsorption data show that at least about 94% (1- about 4%) of the radiocesium was adsorbed after 48 hours contact by samples No. l, 2 and i i-representative synthetic amorphous zeolites obtained by precipitating oxides of Na, Si and A1 0 in the presence of preprecipitated silica obtained from acid decomposition of kaolin clay. These samples were at least as effective in a 48 hour contact time is clinoptilolite, sample No. 4, which was the most effective of the minerals reported in the screening test, and were significantly more effective than the illites.

The data also show that adsorption of radiocesium by the synthetic amorphous zeolites, in accordance with this invention, was substantially complete after standing overnight in contrast with most of the naturally occurring minerals, including the crystalline zeolite (sample No.

4), in which case adsorption was improved by more prolonged contact of the adsorbent with the solution.

, Cesium adsorption of natural and synthetic silicates SYNTHETIC AMORPHOUS ZEOLITES DERIVED FROM KAOLIN CLAY Cesium Adsorption, Base- Percent Surface Exchange Sample No. Composition of Adsorbent Sample Area, Capacity,

7 Mfl/gm. MeqJlOD Sample Sample gm. 1 Standing Standing Overnight 48 hrs.

1 Na:O.AlzO3.8.0SiOz.6HzO I 63 217 94. 9 94.0 2 N2130.AlzO3.9.QSl0z.5H2O 263 230 v 97. 6 97. 2 N810.2AhO3.1l.5SiOg.fiHzO 177 184 (T) 95. 9

NATURALLY QCCURRING MINERALS 4 Clinoptilolito 217 B3. 8 98. 4 5 'te 70. 5 I 78. 9 5 Grundite 70. 1 68. a

a A material supplied under trade name fZeolex 23 and understoodto be made fro'rnkaolin clay by process Y in Us. Patent No. 2,848,346.

b Preparation described above. v s Preparation described above. 4 i 4 American Petroleum Institute Research Project, Reference Clay Mineral No. 35.

' P A type of illite obtained trom'Oak Ridge National Laboratory.v

t Adsorbent did not settle out to give clear supernatant for testing.

All .values subject to a rather uni due to magnitude of countsfrecorded.

iorm absolute error of about 4 units in each percentage adsorption value r 7 We claim: I 1. A method for purifying liquid radioactive waste material which comprises contacting. an aqueous Waste liquid containing radiocesium in solution'with a solid synthetic amorphous hydrous zeolite for a time sufficient to adsorb radiocesium in said liquid on particles of said i for a time within the rangeof about to 48 hours, therezeolite, and separating waste liquid thus contacted from a said zeolite, said zeolite containing 4 to 14 mols of SiO; per mol of Na O and fromabout 1 to 2 mols of A1 per mol of Na O and being obtained by precipitating oxides of sodium, aluminum and silicon in intimate association with finely divided particles of pre-precipitated' silica.

2. The method of claim 1 in which the mol ratio of SiO tO Na 0 in said zeolite is from 8 to 12.

3. A method for purifying an aqueous Waste liquid containing a low level of radiocesium in'the form of a dissolved salt of a mineral acid which comprises contacting said liquid with a solid synthetic amorphous hydrous zeolite for a time sufficient to adsorb radiocesium on particles of said zeolite, and separatingliquid thus contacted from said zeolite, said zeolite containing 4 to 14 mols of SiO per mol of Na O andifrom about 1 to 2 mols of A1 0 per mol of Na O and being obtained by precipitat ing oxides of sodium, aluminum and silicon in intimate association with finely divided particles of pie-precipitated silica.

4. The method of claim 3-in, which the mol ratio of SiO to Na O in said zeolite is from 8 to 12.

5. A method for purifying a substantially neutral spent aqueous waste liquid containing a low level of radiocesium in the form of a nitrate salt and also containing in solution a salt of a nonradioactive cation which comprises contacting said liquid at ambient temperature with particles of a solid synthetic amorphous hydrous zeolite silica.

by to selectively adsorb radiocesium on particles of said zeolite,-and separating liquid thus contacted from said 'zeolite, said zeolite containing .4 to 14.mols of SiO;;

per mol'of'Na O and from about 1 to 2 mols" of A1 0 per mol of,Na O and being obtained by'fprecipitating" oxides of sodium, aluminum and'silicon in intimate association with finely divided particles of pro-precipitated 6. The method of claim 5 in which the molratio of SiO;, to M 0 in said zeolite is from 8 to 12.

7. The method of claim 5 wherein said waste liquid 7 contains sodium ions in excess of ions of radiocesiurn.

OTHER REFERENCES q Removal of Cs Tand-Sr From Aqueous Solution by Ion Exchange on vermiculite, Sammon et a1., At.

Energy Research Estab. (Great Britain) "R4274, 14 pp. (1960), 23873i.

Zeolitic Extraction of Cesium From Aqueous Solutions, Ames, US. At. Energy Comm., HW 62607, 25 pp. (1959)., cited'in; Chem. Abstracts, vol. 54 (1960),

cited: in Chem. Abstracts, vol. .54" (1960),-

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3,167 ,504 January 26, 1965 Walter L. Haden, Jr., et al.

I It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected belo' In the grant, line 1, and in the heading to the printed specification, line 4, name of first inventor, for "Walter L. Hayden, Jr.", each occurrence, read Walter L. Haden, Jr.

Signed and sealed this 15th day of June 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents Patent No. 3,167,504 January 26, 1965 Walter L. Haden, Jr., et al.

I It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

In the grant, line 1, and in the heading to the printed specification, line 4, name of first inventor, for "Walter L.

Hayden, Jr.", each occurrence, read Walter L. Haden, Jr.

(SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents ERNEST W. SWIDE ing 1 Oificer 

1. A METHOD FOR PURIFYING LIQUID RADIOACTIVE WASTE MATERIAL WHICH COMPRISES CONTACTING AN AQUEOUS WASTE LIQUID CONTAINING RADIOCESIUM IN SOLUTION WITH A SOLID SYNTHETIC AMORPHOUS HYDROUS ZEOLITE FOR A TIME SUFFICIENT TO ADSORB RADIOCESIUM IN SAID LIQUID ON PARTICLES OF SAID ZEOLITE, AND SEPARATING WASTE LIQUID THUS CONTACTED FROM SAID ZEOLITE, SAID ZEOLITE CONTAINING 4 TO 14 MOLS OF SIO2 PER MOL OF NA2O AND FROM ABOUT 1 TO 2 MOLS OF AL2O3 PER MOL OF NA2O AND BEING OBTAINED BY PRECIPATING OXIDES OF SODIUM, ALUMINUM AND SILICON IN INTIMATE ASSOCIATION WITH FINELY DIVIDED PARTICLES OF PRE-PRECIPITATED SILICA. 